Furnace element



Patented Feb. 26, 1952 FURNACE ELEMENT Francis E. Bash, Morristown, N. J, assignor to Driver-Harris Company, Harrison, N. J a corporation of New Jersey No Drawing. Application September 23, 1949, Serial No. 117,518

4 Claims. (Cl. 75171) This invention relates to resistor elements and structural parts of furnaces for use at high temperatures in atmospheres of controlled composition. Such controlled atmospheres are obtained by the combustion of fuel, such as, city gas, propane, butane, hydrogen or other means. If a large supply of air is used in the combustion, the resulting gas will contain a limited amount of carbon monoxide and free hydrogen and larger percentages of carbon dioxide, water vapor and 3 mium, with relatively small amounts of other elements. These alloys are known to have a longer life in service in oxidizing atmospheres than any other class of alloys, for the reason that the surface oxide produced at high temperatures acts as a protective coating, which prevents the penetration of further oxidation. The use of these alloys under these conditions results in high economies in operation.

If a limited supply of air is used in the combustion of the hydrocarbon fuel, the resulting gas will be mainly composed of carbon monoxide, free hydrogen, water vapor, and a small percentage of carbon dioxide. Since this gas contains no free oxygen, it acts as a reducing atmosphere due to the presence in excess of carbon monoxide and free hydrogen, which act as reducing agents on most metals with which they come in contact.

There are many furnaces in which reducing atmospheres must be used, for example, in the annealing of iron or nickel or copper alloys or in furnaces for the copper brazing of steel parts. In these situations, it is necessary to maintain a reducing atmosphere to prevent oxidation of the parts which are being annealed or brazed.

It would normally be expected that the nickel chromium or nickel iron chromium alloys, which give such excellent service in oxidizing atmospheres, would have an even more extended life under reducing conditions where oxidation is normally considered to be at a minimum or absent entirely. But when these chromium bearing, nickel alloys were so used they were subjected to a new form of deterioration which rapidlydestroyed the metallic components of the electrical resistors and the furnace parts while leaving the load in the furnace unaffected. The furnace, in consequence, becomes rapidly inoperative and the parts have to be replaced.

This entirely unexpected result arises from the fact that atmospheres which are normally reducing to iron or nickel or copper and to most of their alloys are oxidizing to the chromium component of nickel chromium or nickel chromium iron alloys. The green chromium oxide so formed does not produce a protective surface coating as in oxidizing atmospheres, but the oxidation continues to penetrate in depth until no chromium metal as such remains. The final product is a mixture of green chromium oxide and metallic nickel or nickel and iron. This form of deterioration is referred to here as Green Rot. As a result of this selective oxidation of chromium in what would otherwise be considered as a reducing atmosphere, the chromium alloys are rapidly disintegrated and rendered unfit for further service. The new and unexpected form of deterioration is particularly vigorous in stagnant atmospheres and in temperature ranges between l700 and 1800 F. It may occur in the electrical elements of the furnace and even in other furnace parts. Because of the m gnetic characteristics of the nickel or iron which remain upon oxidation of the chromium, the deterioration of the elements or of the furnace parts can be discovered by testing for magnetic permeability or similar properties.

The character of the attack, herein designated as "Green Rot, is entirely different from the normal corrosion attack observed when chromium bearing alloys are subjected to oxidizing atmospheres. In this latter case the oxide penetration is intercrystalline, that is along the grain boundaries which exist between the grains. The usually accepted explanation is that chromium carbides are precipitated on the grain boundaries, robbing chromium from the metal in the immediate vicinity of the grain boundary and rendering the depleted metal more susceptible to corrosion. The addition of such carbide forming materials as titanium and columbium will prevent this grain boundary deterioration since the carbides of titanium or columbium are much less soluble in the chromium bearing alloy. The migration of carbon to the grain boundaries is thereby reduced to a minimum and the tendency to grain boundary corrosion is eliminated.

The form of corrosion indicated as Green Rot is. however, of an entirely different character. In the first place it is not intergranular, that is between the grains. It is actually transgranular or through the grains. It penetrates the individual crystals and converts all of the metallic chromium to a non-metallic green chromium oxide. Further, it has been observed the final alloy may consist of that when the chromium bearing alloys are carburized, thereby increasing the carbon content on the surface, they are less susceptible to Green Rot attack. This can onlymean that a high concentration of chromium carbide on the surface renders the material less susceptible to attack. The addition of titanium or columbium to the alloy would, under such conditions, be expected to reduce the carbides on the surface and thereby render the alloy more susceptible to Green Rot attack.

I have found, however, that additions of columbium of from 0.2 to 5 percent have quite the opposite effect. No attack of a green rot nature takes place in such chromium bearing nickel alloys in reducing atmospheres at temperatures ranging from 1000 to 2200 F. While the reason for this protective action of columbium cannot be definitely stated, it can at least be concluded that-in the presenceof columbium in chromium bearing nickel alloys, a protective film possibly columbium carbide or oxide is formed on the surface of the alloy which prevents the penetration of those normally reducing components of the reducing atmosphere which are capable of oxidizing the chromium in the alloys under consideration. But whatever the explanation of these phenomena may be, I have proved as an experimental fact that the presence of columbium in these alloys is a specific antidote to green rot and can be used as a means for its elimination.

- When the columbium is added to an 80-20 alloy, the following range of ingredients:-

g Percent Chromium 30- Columbium 0.2 -'.5 Iron 0' 2 ,-Manganese 0 4 Silicon 0 3 Carbon 0 .25 Qther elements a l- 0 .1

Nickel 54.75-89.33

' A preferred range of the alloying elements is as follows:

Percent Chromium 18 -22 Columbium 1 2 Iron .2 2.0 Maganese 0 2 Silicon 1 2 "Carbon .02- .15

Other elements 0 1 Nickel 69.85-79.78

In addition to the. ingredients, set forth in. the

above analyses, the alloys may includethe present of small amounts of other elements which .do not materially affect the above functions of 4 elements but in many instances, it also occurs on structural or other parts of the furnaces which are subjected to the same temperature and atmospheres as the heating elements. I therefore do not limit my invention to the use of such alloy in heating elements but include within the scope of the invention the use of such alloy in the manufacture of all structural parts of such furnaces or other apparatus in which such conditions of temperature and atmosphere are encountered.

I claim:

1. A furnace structural element for use in furnaces having atmospheres which are reducing to nonchromium-bearing alloys but selectively oxidizing to chromium consisting essentially of 10 to 30 percent chromium, 0.2 to 5 percent columbium, 0 to 3 percent silicon and the balance nickel. 2. A furnace structural element for use in fur- 0.to 3 percent silicon and the balance nickel,

said alloy being capable of withstanding selective oxidation of chromium normally occurring when chromium-bearing alloys are exposed to furnace atmospheres which are reducing to non- .chromium-bearing alloys.

4. A furnace structural element for use in furnaces having atmospheres whichare reducing to non-chromium-bearing' alloys but selectively oxidizing .to chromium consisting essentially of 18 to 22 percent chromium, 1 to 2 percent columbium, 1 to 2 percent silicon, and about 70.150

percent nickel, said alloy being capable of withstanding selective oxidation of chromium normally occurring when chromium-bearing alloys are exposed to furnace atmospheres which are reducing to non-chromium-bearing alloys.

FRANCIS E. BASH.

REFERENCES CITED 7 The following references are of record in'th'e file of this patent:

UNITED STATES PATENTS Number Name I Date 2,145,020 Becket et al Jan. 24, 1939 2,174,025 Wise 6t 2.1 Sept. 26, 1939 2,400,255 Pfeil May 14:, 1946 FOREIGN PATENTS Number Country Date Great Britain Sept. 26, 1939 

1. A FURNACE STRUCTURAL ELEMENT FOR USE IN FURNACES HAVING ATMOSPHERES WHICH ARE REDUCING TO NONCHROMIUM-BEARING ALLOYS BUT SELECTIVELY OXIDIZING TO CHROMIUM CONSISTING ESSENTIALLY OF 10 TO 30 PERCENT CHROMIUM, 0.2 TO 5 PERCENT COLUMBIUM, 0 TO 3 PERCENT SILICON AND THE BALANCE NICKEL. 